Micro-electromechanical systems (MEMS) enable miniaturization of engineering systems and tools for a variety of applications due to their ability to create micro-scale systems having high sensitivity and low power consumption. Micro- and nano-scale components for a MEMS device may be fabricated concurrently as an integrated system, or components can be fabricated individually and then incorporated into a MEMS device. Advanced materials and fabrication techniques are required to produce highly precise multidimensional components for use in the MEMS devices.
Bulk metallic glasses are a class of material which can be used to fabricate components for MEMS devices. Bulk metallic glasses (BMGs) are amorphous metals that are rapidly quenched from a molten state to prevent crystal structure formation. A significant factor that determines the glass forming ability of a metal is the critical cooling rate. A sufficiently high critical cooling rate is required to bypass crystallization when cooling from a stable liquid phase in order to form a glass. Once in a glassy amorphous state, it is possible to complete a thermoplastic forming of BMGs at comparatively low temperatures using simplistic forming processes compared to traditional metals that result in near-net shaping. Thermoplastic forming is achieved by elevating the temperature of the BMG above the glass transition temperature followed by the application of pressure, which causes the BMG to conform to the shape of a mold patterned with the desired final features. Methods of thermoplastic forming of BMGs include but are not limited to hot embossing, blowmolding, and imprinting.
The complexity and precision of BMG components for use in MEMS devices is dependent of a mold structure used as part of the BMG processing. The methods of forming mold structures for metallic glass components with micro and nano-scale features offer the ability to produce a variety of highly-precise two-dimensional (2D) variation, but the resulting features are limited to a constant feature size, or minor variation of the 2D feature such as a taper angle, in the third dimension since the molds generally consist of a single substrate.
Referring now to FIG. 1a-1b, a cross-sectional view of a typical mold used to produce bulk metallic glass microscale components as known in the prior art is illustrated. A typical mold consists of a substrate 101, generally made of silicon. The top surface of substrate 101 contains a cavity 102 that is formed into the surface of substrate 101 using patterning processes known to those skilled in the art, including but not limited to photolithography and reactive ion etching, e.g., a RIE etching of the silicon substrate material. FIG. 1B shows a top view along line A-A (of FIG. 1A) illustrating cavity 102 formed in substrate 101. Due to the limitations of forming mold patterns on a single surface of a substrate, the molds existing in the prior art are limited to have a single two-dimensional feature as depicted in FIGS. 1A and 1B, having only features with a larger diameter than the nominal dimensions of cavity 102 on the top exposed surface of the mold. Mold cavity 102 is then filled with the desired high-temperature filling material, such as a bulk metallic glass alloy, by thermoplastic forming of the BMG into substrate 101. After removal of the mold substrate, the resulting BMG component features variation in two dimensions but only an extrusion of the 2D feature with no additional variation in the third dimension.
Mold structures with increased complexity can be formed by bonding stacked substrates to form a bonded mold. However, several limitations with bonded mold structures prohibit the use of metallic glass or other filling materials requiring elevated processing temperatures. For example, polymer-based adhesive materials that have been previously demonstrated as a way of bonding stacked silicon substrates induce thermal budget limitations on the filling materials that may be used. Since the processing temperature of zirconium-based and other metallic glass systems exceeds 400° C., a polymer-based adhesive would not be conducive for use in a bonded mold for many metallic glass forming applications. Additionally, the accuracy of features able to be produced using polymer-based adhesives to create a bonded mold is not precise due to the flow ability of polymer bonding agents during the substrate bonding process, which is typically completed using thermocompression bonding. The application of pressure at elevated temperatures required for successful thermocompression bonding of adhesives causes shifting of the substrates with respect to each other that can result in misalignment in excess of several microns. The flowability of the adhesive also causes local distortion of any patterned feature, which prohibits net-shape forming of feature with sharp angles or precise dimensions.
In the case of both 2D and 3D molds used for thermoplastic forming of BMGs, removing any residual BMG overburden from the top surface of the mold for highly precise micro and nano-scale parts remains a challenge. Removal methods known in the art such as grinding, polishing, and hot scraping may be applied as post-processing methods for larger-scale BMG components. However, for micro and nano-scale BMG components with highly precise features, the comparably large cross sectional area of the BMG overburden with respect to the final component exposed features can result in localized high forces that may cause delamination or distortion of the final BMG component during the overburden removal process.